The present invention relates to vehicles including a variable gear ratio steering device and an electric power steering apparatus.
Vehicles including a variable gear ratio steering device and an electric power steering apparatus have been widely known. The variable gear ratio steering device influences on a steering system of the vehicle and continuously varies the steering angle ratio (also referred to as xe2x80x9ctransmission ratexe2x80x9d or xe2x80x9creduction gear ratioxe2x80x9d) or the ratio of the steering angle of the road wheels to the steering wheel angle. The variable gear ratio steering device sets the steering angle ratio to a greater value in a low speed range, at which a steering operation with a large steering input is often necessary, so that the turn or displacement of the steerable road wheels is relatively large with respect to a relatively small steering input. This is so-called quick state. On the other hand, the variable gear ratio steering device sets the steering angle ratio to a smaller value in a high speed range, at which a steering operation with a large steering input is not required, so that the turn or displacement of the road wheels is relatively small with respect to a relatively large steering input. This is so-called slow state. Meanwhile, the electric power steering apparatus assists a driver""s steering wheel torque by making direct use of the driving force of an electric motor, so as to ease the effort required for the steering wheel operation. Therefore, the driver is not required a laboring steering operation.
The variable gear ratio steering device and the electric power steering apparatus are provided with an electric motor, respectively, which is driven by a power supply from a battery. When controlling the drive of each electric motor, in the variable gear ratio steering device, the steering angle ratio is increased or decreased in accordance with a vehicle speed, and in the electric power steering apparatus, the assist steering wheel torque is increased or decreased in accordance with the driver""s manual steering wheel torque input.
However, in these vehicles, in order to prevent an engine stop due to decreased voltage of the battery, a consideration is made such that the assistance of the electric power steering apparatus is stopped and the variable control of the steering angle ratio (steering ratio characteristics) with the variable gear ratio steering device is stopped.
However, since the variable gear ratio steering device ensures a large displacement of the road wheels with a small steering input during the quick state in a low speed range, the steering operation cannot be facilitated without assistance of the electric power steering apparatus. If the voltage of the battery lowers when the variable gear ratio steering device is in the quick state, and the assistance of the electric power steering apparatus is lowered or stopped and the variable control of the steering angle ratio with the variable gear ratio steering device is stopped, the movement of the steering wheel becomes heavy. This is not preferable. Meanwhile, in order to prevent the engine stop, decreasing the voltage of the battery has to be restricted. Further, these fail-and-safe actions are preferably carried out without giving an uncomfortable feel to the driver.
In view of the above, the present invention seeks to provide a vehicle, which solves the drawbacks of the prior art.
According to a first aspect of the invention, there is provided a vehicle including a variable gear ratio steering device and an electric power steering apparatus, the variable gear ratio steering device influencing on a steering system of the vehicle and varying a ratio of a steering angle of steerable road wheels to a steering wheel angle by driving an electric motor in accordance with a vehicle speed, and the electric power steering apparatus influencing on the steering system of the vehicle and providing an assist steering wheel torque by driving an electric motor in accordance with a manual steering wheel torque, the vehicle comprising: a power supply voltage detector for detecting a voltage of a power supply; and a control unit for controlling the electric motor of said variable gear ratio steering device in accordance with the detected voltage, wherein, when the detected voltage decreases to a certain voltage or lower, the control unit brings the variable gear ratio steering device into a slow state and thereafter stops the variable gear ratio steering device.
In such a construction, when the voltage of the power supply decreases to a certain voltage or lower, the variable gear ratio steering device is stopped, thereby saving the life of the power supply or recovering the power supply. This can prevent or restrict occurrence of a trouble, such as an engine stop. It should be noted that when the voltage of the power supply lowers and so does the assist steering wheel torque of the electric power steering apparatus, in some conditions of the variable gear ratio steering device, a difficulty will a rise in a steering operation. Therefore, the variable gear ratio steering device is preferably brought into the slow state (dull) where the steering angle ratio is small, such that the steering operation is carried out with decreased assistance of the assist steering wheel torque. Herein, xe2x80x9ca certain voltagexe2x80x9d is set higher than the voltage that respective control means (computers) for controlling vehicle-mounted equipment can be operated without any troubles and that is extremely low in comparison with the normal state of use. The certain voltage also indicates the critical voltage, below which the variable gear ratio steering device and/or the electric power steering apparatus may not be operated normally, otherwise the engine may stop when starting the variable gear ratio steering device and/or the electric power steering apparatus. The certain voltage is for example 9.5V.
According to a second aspect of the invention, said control unit reduces assistance of the assist steering wheel torque from the electric motor of said electric power steering apparatus when the detected voltage decreases to the certain voltage or lower, and said control unit stops the assistance of said assist steering wheel torque when the voltage further decreases to a lower limit voltage which is set to be lower than the certain voltage.
In such a construction, assistance of the assist steering wheel force (assist steering wheel torque) of the electric power steering apparatus is reduced when the power supply voltage becomes the certain voltage or lower, and the assistance is stopped under the lower limit voltage, thereby saving the life of the power supply or recovering the power supply. Therefore, reduction of the power supply voltage can be prevented in a reliable manner. The certain voltage may be the same as that recited in the aforementioned construction. The lower limit voltage is defined as a low voltage, in which for example electric equipment may not be actuated but at least the engine can manage to move. The lower limit voltage is for example 8V.
According to a third aspect of the invention, said control unit carries out assistance by decreasing said assist steering wheel torque in accordance with a value of the detected voltage when the detected voltage is between the certain voltage and the lower limit voltage.
In such a construction, assistance is reduced in accordance with the detected voltage, and thereby the power supply is not subject to a heavy load. Further, except that the voltage lowers instantly and abruptly, the driver does not experience any uncomfortable feel even if the driver carries out a steering operation while the voltage lowers. Also, except that the voltage recovers instantly, the driver does not experience any uncomfortable feel during the steering operation even if the voltage recovers and increases.
Preferably, control means is provided for restricting electric power consumption of the equipment which is unnecessary to drive the engine or which does not affect safety of the vehicle, such as an air conditioning system or a car audio system. The control means controls an operation of such equipment based on the detected voltage such that when the detected voltage decreases to the certain voltage or lower, the operation amount of the equipment is preferably reduced or alternatively the equipment is stopped. As mentioned above, a cooperative control of the variable gear ratio steering device and the electric power steering apparatus with other equipment ensures a smooth steering operation as well as an improved battery life extending operation (battery recovering operation).
Further, in the a fore-mentioned constructions, preferably the control unit determines under voltage when the detected voltage decreases to the certain voltage or lower continuously for a certain period of time, and then as a fail-safe action, the control unit reduces assist of the assist steering wheel torque and brings said variable gear ratio steering device into a slow state and thereafter stops said variable gear ratio steering device. Preferably, when the detected voltage decreases to the certain voltage or lower continuously for the certain period of time, the control unit cooperatively controls said variable gear ratio steering device and said electric power steering apparatus with other equipment. This is because the voltage lowers due to noise or voltage fluctuation even in the normal state of use, and a fail-and-safe action is not always required. The lowered voltage due to noise or voltage fluctuation recovers after a relatively short period of time, such as less than 0.2 or 0.5 seconds. Therefore, a judgement can be made as to whether or not a fail-and-safe action is required by setting the certain period of time. An unnecessary fail-and-safe action is avoidable when the voltage lowers in the normal state of use.